This invention relates to an improved waterflooding process for recovering oil from a subterranean oil-containing formation.
Waterflooding processes have long been used in the recovery of oil from oil-containing formations or oil reservoirs. Generally, in accordance with these processes, at least an injection well and a production well are provided which communicate with the reservoir and water is injected thereinto via the injection well and oil is produced therefrom via the production well. Many modifications of the basic waterflooding techniques have been employed. These modifications include the use of various chemicals and materials in the water injected into the formation to improve the recovery of oil therefrom. Surfactants have been included to reduce the interfacial tension between the injected water and the reservoir oil and thereby facilitate the recovery of the oil from the reservoir. Waterflooding processes utilizing surfactants are commonly referred to as low tension waterflooding processes. Materials and in particular polymers have been employed in the water to increase the viscosity thereof and thereby improve the efficiency of the water in displacing the oil from the reservoir.
In the carrying out of a surfactant or low tension waterflooding process, it is common practice to inject into the oil-containing formation a slug of a surfactant and follow this with a slug of thickened water to more efficiently drive the surfactant through the formation and thus improve the sweep efficiency thereof. These slugs are then usually driven through the formation by the injection of a driving fluid which is oftentimes recovered formation brines.
In U.S. Pat. No. 2,802,785 there is described a waterflooding process in which the flooding medium comprises an aqueous solution of certain surface active agents which are amphoteric in nature, i.e., which ionize in such manner that both anionic and cationic functional groups are an integral part of the molecule. In addition to their surface active properties, these agents have strong bactericidal action and do not precipitate alkaline-earth metal salts. The bactericidal surface active agents employed are the acid salts of amido-acids having the general formula:
R--conh--(ch.sub.2 ch.sub.2 nh).sub.x --R'--COOH
wherein R represents an alkyl group containing from about 10 to 22 carbon atoms, x represents a numerical value from 0 to 4 inclusive, and R' represents a divalent aliphatic radical containing from 1 to 3 carbon atoms.
In a paper by W. R. Foster entitled "A Low Tension Waterflooding Process", Journal of Petroleum Technology, Vol. 25, February 1973, pp. 205-210, there is described a waterflood process wherein three slugs of water with different chemical compositions are injected into a formation, which slugs are denoted as a protective slug, a surfactant slug, and a mobility control slug. The protective slug is an aqueous solution of sodium chloride injected for the purpose of screening the low tension surfactant of the surfactant slug from the reservoir brine and base exchanging the reservoir solids -- replacing magnesium and calcium with sodium ions. The surfactant slug contains the same sodium chloride content as the protective slug and contains a selected petroleum sulfonate. The primary purpose of the surfactant slug is to reduce the interfacial tension between the oil and the water to the order of 0.001 dyne/cm. The mobility control slug has considerably less sodium chloride content than the other two slugs and contains a water soluble biopolymer in a sufficient concentration to provide a locally stable mobility condition at the rear of the oil bank. The sequence of three slugs can be driven through the formation by the original reservoir brine.
In U.S. Pat. No. 3,984,333 there is described an oil recovery process in which an aqueous solution is injected into a reservoir to displace oil through the reservoir and wherein the aqueous solution is thickened with a substantially linear block copolymer containing both water soluble and water insoluble polymer chains. Suitable block copolymers comprise blocks of sulfonated polystyrene mixed with blocks of hydrogenated-polyisoprene or hydrogenated-polybutadiene.
In U.S. Pat. No. 3,085,063 there is disclosed an invention which is concerned with waterflooding and more particularly with the use of an improved class of water thickening agents wherein aldehyde solutions are used in the waterflood. The preferred water thickening agents are selected from the class of compounds comprising sulfonated polymers. Desirable polymers are polyvinyl aromatic sulfonates, as, for example, polyvinyl toluene sulfonates. Other water thickening agents may comprise sulfonated polymers as, for example, polyvinyl toluene sulfonates, polystyrene sulfonates, or substituted polystyrene sulfonates. Still other desirable water thickening agents may be secured by copolymerizing vinyl aromatics such as styrene, vinyl toluene, vinyl naphthalene, and the like, with maleic anhydride.
In U.S. Pat. No. 3,116,791 there is described a waterflooding process wherein there is employed a viscous aqueous solution of water soluble alkylene oxide polymer. The alkylene oxide polymers taught for use are the water soluble polymers of ethylene oxide, a propylene oxide or a butylene oxide. In addition to these alkylene oxide homopolymers there may also be employed copolymers of such alkylene oxides with other epoxy compounds, such as the epoxybutanes, butadiene oxide, and styrene oxides.
In U.S. Pat. No. 3,945,437 there is described a surfactant waterflooding process wherein an aqueous anionic surfactant system containing an aromatic ether polysulfonate is used for displacing oil from an oil reservoir. The aromatic ether polysulfonate improves the tolerance of the system toward salts of monovalent and multivalent cations and causes emulsions formed between the oil and the oil displacing system to be less viscous and less stable.
In U.S. Pat. No. 3,747,676 there is disclosed a waterflooding process wherein there is included in the water injected into an oil reservoir methylolated polyacrylamides formed by reacting high molecular weight polyacrylamides with aqueous solutions of formaldehyde.
In U.S. Pat. No. 3,583,486 there is described an improved waterflooding technique wherein there is included in the water injected into an oil-bearing formation an additive which has both surface active and viscosifying properties, which additive is an ethoxylated condensation product of a phenol and formaldehyde.
Various problems have been encountered in carrying out such low tension waterflooding processes. One problem involves the reactions which commonly take place between divalent ions, primarily calcium and magnesium ions, found in formation brines with the chemicals in the surfactant or mobility slugs to form insoluble precipitants which reduce the permeability of the formation. Another problem revolves around driving the surfactant slug uniformly through the formation to obtain good sweep efficiency and thus efficiently displace the oil from the formation to production wells.